The number one rule about weight loss may not even be true

When is a calorie not a calorie? No, this isn't the intro to some geeky joke. Recently scientists have been exploring the idea that all calories are not created equal when it comes to how they act in the body (and how they end up on your hips).

Recent research, reported in the International Journal of Obesity, examined the validity of the gold standard of diet advice: If you burn more calories than you take in, then you'll lose weight. And what they found may surprise you (or not, depending on your personal experience with the weight loss wars). Most weight loss studies today rely on personal reports of energy intake (food eaten) and physical activity energy expenditure (exercise) to determine whether or not a diet works, but this study found these to be "decidedly inaccurate" and warned that continued reliance on these measures could seriously misguide health care policies, future research and clinical judgment.

For years, dieters have been told that reaching a healthy weight really is as simple as a third-grade math equation. And for just as long, some people have been saying that despite their best efforts to slash calories and increase exercise, they're just not dropping pounds. The problem, according to the researchers, may not just be in the math itself but in how we measure calories taken in and expended.

The first problem with the "calorie as king" method is that nutritional counts on labels show how much energy it takes to burn up a food in the lab, but this doesn't take into account how different foods react in the body. For example, subbing a "low-cal" cheese spread for avocado on your sandwich may save you calories, but the carbs and sugar in the spread will spike your insulin, causing your body to retain more of the calories and store them exactly where you don't want them. Sound crazy? A separate study from earlier this year showed that volunteers who ate a "junk food" meal burned less calories afterward than when they ate a healthier meal with exactly the same number of calories.

Eating nutrient-dense foods like fruits and vegetables helps promote feeling full, while processed foods have been shown to increase cravings and hunger, so even though that avocado has more calories, you still may end up eating less overall because you'll feel full faster. In addition, the healthy fats and minerals have numerous health benefits that no factory food can match.

The estimation problem happens on the other end as well: Exercise machines have been shown to wildly overestimate calories burned, sometimes by up to 80 percent. That stair climber might say you've burned 800 calories in half an hour, but the internal computation may be based on a 6-foot-6-inch Olympic swimmer rather than the average woman. A heart rate monitor programmed to your exact settings can give you a more accurate measure, but it's still an estimate, because people have different baseline metabolisms. So if you're relying solely on the math to tell you how much food you've "earned," then it's no wonder it's not working.

But perhaps the biggest problem with the calories-in-calories-out school of thought is how little it teaches people about what their bodies actually need. It can be hard to focus on what "feeling full" really feels like if you're super focused on staying under a certain number. And if you use exercise only as a "punishment" for eating too much or as a way to earn your food, then you miss out on all the mental health and other benefits of moving your body in a joyful way.

Counting calories may be beneficial in the short term to help people learn about portion sizes, but as this research finds, it's not a great long-term solution. And even if it did work exactly as it should, we need to find a way to put the focus back on total health and not just weight.